Synthesis and Characterization of Nickel Particles by Hydrogen Reduction Assisted Ultrasonic Spray Pyrolysis(USP-HR) Method

Spherical nickel particles were prepared by hydrogen reduction assisted ultrasonic spray pyrolysis (USP-HR) method using nickel chloride solution without any additives. Thermodynamic of the hydrogen reduction of the nickel chloride were studied by FactSage software. Particles were obtained at 800°C reaction temperature by hydrogen reduction of aerosol droplets under H2 flow. The effects of the precursor concentration on the particle size and morphology were investigated by scanning electron microscopy. Results showed that nickel particle sizes were decreased from 630 to 270 nm by reducing solution concentration, and also narrower size distribution was obtained using lower concentrated precursor. Nickel particle sizes were theoretically calculated and results indicated that there was a slight difference in the particle sizes compared to experimental values.


Introduction
Fine powders of the transition metals especially iron, cobalt, nickel, copper and their alloys have been drawn increasing attention for several years due to their novel magnetic, electrical and catalytic features.In the case of nickel particles, the relation between the grain size of Ni nano-crystals and their magnetic properties has been studied in the past decade.The reason of the attention on nickel fine particles is the effect of particle/cr ystalline size on their physical properties which providing an opportunity to use them in various practical applications such as catalysts, electrodes in electronic products, magnetic fluids and high density recording media [1][2][3][4][5][6] . Vrious techniques have been developed for the synthesis of submicron particles such as mechanical alloying which is the well known top down method, beside that microemulsion methods, polyol process, chemical vapor deposition, laser pyrolysis, gas deposition, microwave plasma, flame spraying, and spray pyrolysis are chemical base bottom up methods [4][5][6][7][8][9] .Among them, ultrasonic spray pyrolysis (USP) method is used for preparation of spherical non-agglomerated ultra fine particles in controlled chemical composition, size and crystallinity, which are suitable for direct application or fabrication of high technology materials [10][11][12] .It is a versatile method to produce metallic, alloy and metal oxide particles in various size and morphology.In USP method, spraying is performed by applying high frequency ultrasound to the precursor solution that forms aerosols with constant droplet size, which depends on the characteristic of the liquid and the frequency of the atomizer.Particle formation occurs from the reduction or thermal decomposition of the aerosol droplets [12][13][14][15][16] .Although several studies were reported on the metallic particle production by USP method such as Co 12) , Fe 14) , FeNi 15) , FeCo 16) and Ag 17) , there is still a lack of nickel particle production, which is an important industrial material.Stopic et al. 18) prepared nickel powders by USP method using aqueous solutions of NiCl2 at 900 and 1000℃ under H2-N2 (1:5) mixture gas atmosphere.They reported that pure Ni particles could only be produced at 1000℃ with H2-N2 gas mixture under 20 sec droplet residence time conditions. Alsothey observed NiO traces in Ni particles obtained at 900 ℃.Besides, Stopic and co-workers 19) 22) reported the production of spherical Ni particles by a large scale spray pyrolysis process with two continuous reactors from nickel nitrate and nickel acetate as a starting material using nitrogen gas containing 10% H2.In their process, the temperature of the first reactor was changed from 300 to 900℃, and the temperature of the second reactor was controlled at the range 900 to 1400℃.
In this research, we investigated the production of the nanocrystalline nickel particles by a simple process using nickel salt without any additives.Spherical and dense Ni particles were prepared by hydrogen reduction assisted ultrasonic spray pyrolysis (USP-HR) technique using nickel chloride aqueous solution at 800℃ under only H2 flow.Also, effects of the corresponding solution on the particle size and morphology were studied and compared with theorical values.

Experimental
Ni particles were synthesized by hydrogen reduction assisted ultrasonic spray pyrolysis method using nickel chloride as a starting material.Nickel chloride hexahydrate (NiCl2.6H2O)was dissolved in distilled water in desired amounts to prepare precursor solutions and concentrations of the solutions were 0.

Thermodynamic analysis of hydrogen reduction
The reaction for the formation of metallic nickel from nickel chloride described as in Eq. 1.The thermodynamic analysis was done using Fact Sage TM software in the temperature range of 50 -1200 ℃, shown in Fig. 1.
The values of Gibbs free energy (ΔG°) for the Eq. 1 at the temperature range up to 1000 ℃ confirm the possibility of formation of nickel from NiCl2 by hydrogen reduction.Although Gibbs free energy is always negative between the 0 -1200℃ temperature range, it increases through the positive values at elevated temperatures.It was supposed that determined reaction time was sufficient for the transformation of droplets to metal particle in the hydrogen atmosphere.

X-ray analysis of nickel powder
XRD pattern of the nickel par ticles produced from 0.8 M solution is shown in Fig. 2. The peaks at 44.61 °, 51.78° and 76.80° were referred to (111), ( 200) and (220) diffraction planes of nickel, respectively.Results showed that pure nickel in face centered cubic crystal structure were produced without oxidation.Also, any peaks due to the incomplete reduction of the starting material such as NiCl2 was not observed.Crystalline size of the sample was calculated by Scherrer Equation using XRD data, Eq. 2 defines a simple relationship between crystalline size, and peak width.In this equation K is constant, the value of which is between 0.85, and 0.9; λ is the wavelength of the X-ray (Cu Kα1 = 1,541874 Å); B is the width (in radians) of the peak due to size effect; θ is the Bragg angle; and t is the particle size.In the crystallite size calculation instrumental broadening was taken into account to obtain accurate size.The crystallite size of nickel particles prepared using 0.8 M precursor solutions was 89 nm.Particles prepared in all concentrations had spherical shape morphology and smooth surface.Ni particles sizes showed reducing trend by decreasing of the corresponding solution concentration.The mean particle sizes for nickel powders produced using 0.8, 0.4 and 0.04 M solutions were 630, 495 and 270 nm, respectively.Moreover the decreasing of the solution concentration increased the size uniformity of the products and narrower particle size distribution was obtained by lessen of the precursor concentration.Although particle size range was between 100 and1130 nm for particles obtained using 0.8 M solution, size distribution was narrowed between 50 and 550 nm using 0.04 M precursor solution.

Theoretically particle diameter
The relation between the droplet diameter and the frequency of the ultrasound source was studied by Jokanovic et al. 23).The mean diameter of the aerosol droplets can be determined by the given equation.
In Eq. 3, D is the mean droplet diameter; γ is the surface tension of the solution; ρ is the density of the solution; and f is the frequency.Thus, droplet diameter was calculated 3.49 μm by Eq. 3 using the parameters of this study (γ:72.9･10 - Nm -1 ; ρ:1 g cm -3 ; f: 1.3 MHz).
The nickel particles diameters produced by USP-HR    method were determined by Eq. 4. For Eq. 4, these were assumed that precursor concentration was homogeneous in all droplets and the aerosol droplets transformed to fully dense nickel particles.
Dp=D(Cprecursor .MNi / Mprecursor .ρNi) 1/3 (4) In Eq. 4, Dp is the particle diameter; D is the droplet diameter; Cprecursor is the concentration of the solution, ρNi is the density of nickel, MNi is the atomic weight of nickel and Mprecursor is the molecular weight of the precursor.Fig. 4 shows the change of the calculated particles sizes by the increasing concentration.The theoretical particle diameters for 0.8, 0.4 and 0.04 M precursor were calculated as 611, 480 and 220 nm, respectively.

Discussion
The experimental obtained mean par ticle sizes and theoretical calculated particle sizes are given in Table 1.The possible reason, why the experimental particle sizes were slightly bigger than the theoretical calculated values, was the insufficient sintering and densification of the particles due to less residence time.Also, the highest size difference between theoretical and experimental results was obser ved for particles obtained by 0.04M solution.It showed that the particles produced using lower precursor concentration had slower densification rate probably due to the less amount of nickel content in the same droplet size.
The active particle formation mechanism for nickel par ticle production by USP-HR method was one droplet to one particle transformation as proposed in Co particles production by Gurmen et al 12) and in our previous study about the iron particles production 14) .
The steps of the one droplet to one particle mechanism for nickel particle production were shrinkage of the droplets due to the evaporation of the solvent in the entrance of the heated zone, reduction reaction of metal salt by hydrogen and nucleation of primary particles (nanoparticles), and finally secondary particles (submicron range particles) formed by sintering and densification of the nickel nanoparticles.The explained mechanism is well suited with theoretical approach and experimental obtained mean particle sizes.Besides, determined cr ystallite size of the nickel particles (89 nm) confirmed the formation of primary particles.

Fig. 4
Table 1 Comparison of the experimental obtained and calculated particle sizes On the other hand, one droplet to one par ticle mechanism has a lack to explain the existence of the smaller/nanoparticles.The one of the possible reasons is the fragmentation of the droplets after nucleation of the primary particles due to collisions.Thus, nanoparticles dispersed and some of them sintered with each other or other droplets which cause the formation of the smaller and bigger particles.

Conclusion
Ni particles were produced by hydrogen reduction assisted ultrasonic spray pyrolysis (USP-HR) method using nickel chloride salt as a starting material without any additives.Ni particles were prepared in the submicron size range in spherical morphology at 800 ℃ reaction temperature.Results showed that decreasing of the corresponding solution concentration caused to not only reduce the mean particle size from 630 to 270 nm, but also narrow the size distribution.

Fig. 3
Fig.3 investigated † Accepted: July 27 th , 2011 solution using hydrogen and argon gas mixture at a residence time of 19 sec.They showed that obtained nickel particles had not spherical morphology and fully densified until 850℃.Recently, Yung et al.
Ramine Baghai Instrumentation, with a frequency of 1.3 MHz).Then, the obtained aerosol droplets were carried into the horizontal quartz reactor by gas flow.Hydrogen was used without mixing any inert gas in the experiments as a carrier/reducing agent in 1.0 L/min gas flow rate.Reduction process took place in the quartz reactor (0.25 m heated zone, and 0.02 m diameter) occupied in the electrically heated furnace (Naber therm, Germany) at 800℃.The residence time of the droplet/particle was about 4.7 sec, which was calculated by taking into account the ratio of the volume of the reaction zone and the carrier gas flow rate.Reaction products were collected in the washing bottles which connected to the outlet of the quartz reactor.X-ray diffraction (XRD) pattern was recorded by Siemens D5000 X-ray diffractometer with Cu Kα radiation (λ= 1.54187 Å, 2θ range 30-80°, 2θ step of 0.016 ℃ and time per step 0.2 sec) to determine the crystal structure of the particles.XRD data was also used to calculate the cr ystalline size of the particles by Scherrer formulation.Scanning electron microscopy (SEM) images of the products were taken by Jeol JSM 7000F FE-SEM.Particle size and size distribution were determined from SEM images by Leica Image Manager.All the clearly obser ved particles on the SEM images were taken into account in these analyses.
8, 0.4 M and 0.04 M. Nitrogen with 1.0 L/min flow rate was used to create an inert atmosphere prior to and after the reduction process due to the safety regulations.Corresponding solution was atomized a high frequency ultrasonic nebulizer Pyrosol 7901 (